A transfer press of the multiplunger type is generally used for encapsulating electronic components, in particular semiconductors and integrated circuits (IC), in a synthetic material, usually a thermosetting plastic. For this purpose, a strip comprising the relevant IC crystals and consisting of several lead frames is positioned in the mold cavities, upon which the mold is closed. The crystals are connected to the lead frames by gold wires, which are usually provided via a bonding process, e.g., ultrasonic weld. A pressed pellet of the synthetic material to be connected to the frames is introduced into each plunger pot of the mold, which has been heated in the interim. The pellets are heated by the hot mold walls and start melting. During the further heating-up and fusion of the pellets and within a certain time, called transfer time, the plungers introduced into the plunger pots now press the molten synthetic material into the mold cavities, where the material is cured. Then the mold is opened and the strip of cohering encapsulated components is removed from the mold, after which the individual encapsulated components are finally cut from the strip.
The pellets, which are usually cylindrical in shape, may have varying dimensions, depending on the dimensions of the components to be encapsulated, the number of bolt cavities to be filled by a single pellet, the dimensions of the gates, etc. In practice, pellets are frequently used having a diameter of 6 to 18 mm and having a diameter/length ratio of between 1 and 1.7; subject to the dimensions of the products to be encapsulated. The pellets may exhibit length deviations of approximately .+-.1 mm.
European Patent Application 0 070 320 discloses a device as described in the opening paragraph in which the pellets are pressed and the mold cavities are filled by means of plungers which are loaded by springs.
This known device has the disadvantage that the force to be transmitted by a single plunger is limited and equal to the total force applied divided by the number of plungers. If a pellet having excess length is present in one of the plunger pots, the spring of the relevant plunger is compressed. This increases the pressure on the pellet, which results in a better contact of the synthetic material with the walls of the plunger pot, the mold, and the gates. Owing to this improved contact, the pellet or the not yet molten core of the pellet will be quickly heated up, by which the viscosity of the synthetic material will quickly fall and the compressed spring will relax with a jolt. This sudden relaxation of the spring results in a too high uncontrolled displacement of the relevant plunger, which in its turn results in too high injection speeds of the synthetic material, which may increase to such an extent that the components are damaged. Thus the gold wires may become deformed (wire sweep) or fractured, or the weld bond connections may be broken.